The present invention concerns the preparation of coatings thin films, which can be used to protect their substrates from wear and corrosion, for functional electronic and optical applications, and for decorative purposes.
Coatings and thin films have many technical applications including protecting surfaces from wear and corrosion, providing optical and electronic functions, and improving the appearance of components. Many methods are commonly used, including in particular the family of methods known as physical vapor deposition (PVD), which includes such methods as sputtering, thermal evaporation, and ion plating. One PVD method, known variously as vacuum arc deposition (VAD) or cathodic arc deposition, was introduced in the late 19th century by A. Wright, and T. A. Edison (U.S. Pat. No. 484,582, 1892), and more recently by Wroe (U.S. Pat. No. 2,972,695, 1961), Kikuchi et al. (Japan J. Appl. Phys. 4, 940, 1965), Snaper (U.S. Pat. No. 3,625,848, 1971), and Sablev et al. (U.S. Pat. Nos. 3,793,179 and 3,783,231, 1974). In its most common form a high current electrical discharge is maintained within a vacuum vessel between a source cathode and a passive anode. A natural concentration of current at the minute areas of the cathode, known as cathode spots, causes intense melting, evaporation and ionization of the cathode material, producing an energetic plasma beam, which upon impinging on a solid surface will generally form a coating of the cathode material. The reaction force of the plasma jet on the minute liquid pool of the cathode also produces a spray of microscopic liquid droplets, which may contaminate the coating. In some applications the inclusion of these droplets, known as macroparticles (MPs), is tolerable. For other applications, Aksenov (Pribory I Teknika Eksperimenta, No. 5, 236, 1978) has shown how to filter the droplets from the plasma stream by using a magnetic field to guide the plasma beam around an obstruction, such as a quarter torus duct, which prevents line of sight contact between the cathode spots and the substrate. Generally, a significant portion of the plasma generated by the cathode spots is lost in this procedure. Sanders (J. Vac. Sci. Technol A6, 1929-1933, 1987) produced a MP-free vapor by using a cathode spot plasma source to bombard a target of the same material as the cathode with energetic ions, and then utilizing the material sputtered from this target to form a coating. Generally, the electrodes used in cathode spot arcs do not reach a high temperature, except locally at the cathode spots. Typically the electrodes are either cooled, or operated with short pulses, to prevent excessive heating.
A different form of vacuum arc was introduced by Dorodnov et al. (Sov. Tech. Phys. Left. 5, 418-9, 1979; Sov. Phys. Tech. Phys. 26, 304-315, 1981) and Vassin et al. (Sov. Tech. Phys. Lett. 5, 634, 1979), in which either the anode or the cathode is thermally isolated, and thus during arcing reaches a sufficiently high temperature so that significant evaporation occurs over a large portion of its active surface. This vapor may dominate the arc, and according to Dorodnov, under certain circumstances, namely when the vapor pressure is greater than 10 Torr, the cathode spots may extinguish, and with it the production of MPs. Thus, what we call the hot electrode vacuum arc (HEVA) can serve as an efficient coating source without MP contamination. Ehrich et al. (Proc. 8th Int. Conf. On Gas Discharges and Their Applications, September 1985, Oxford, p. 596) teaches another hot anode vacuum arc (HAVA) source in which the cathode spots do not extinguish. Ehrich overcomes the problem of MP contamination by the use of shields and a judicious choice of substrate location, so that there is a line of sight between the hot anode and the substrates, but no line of sight between the cathode and the substrates. While solving the MP problem, the various HEVA sources have several disadvantages with respect to cathodic arc sources. The most serious is that usually the evaporating material from the hot anode is generally molten (with the exception of some materials (e.g. Cr) for which significant sublimation occurs below the melting point.). The presence of large quantities of molten material must typically be held upright in some form of cup or crucible. This limits the orientation of the hot electrode, and often metallurgical interactions between the molten source material and the crucible/electrode material can limit the life of the hot electrode and contaminate the coating. Furthermore, different evaporation rates can effectively prevent coating with alloy materials from a single source.
The present invention relates to a new method and apparatus for depositing coatings and thin films, which uses a vacuum arc sustained between a consumable cathode and a non-consumable hot anode. Material is evaporated from the consumable cathode. During the beginning of the arc, the anode, constructed from a refractory material, is heated to a sufficiently high temperature that any material reaching it will be quickly re-evaporated. In one embodiment, an operating mode is established in which copious quantities of cathode vapor is emitted without MP emission, or with reduced MP emission. In another embodiment, the hot anode also serves as an obstacle blocking MPs from alighting on the substrate, while pressure gradients or magnetic fields guide the vapor around the anodic obstacle towards the substrates. Substrates intercepting this vapor are coated without MP contamination.